RAID config with disk utility

Since no one else has replied yet, I'll chime in with my suggestions ...

First, some suggested reading:

Software vs. Hardware Based RAID for External Drive

macOS Disk Utility Can Create Four Popular RAID Arrays

Create a disk set using Disk Utility on Mac (be sure to select the drop-down for your version of macOS)

Disk Utility will show you only those disks that can be added to the RAID array. If a USB-connected drive is in the list then it's eligible to be used. But I vote for both disks to be connected the same way (specifically via Thunderbolt if you have it).

I would definitely NOT mix connection types with a RAID array, as it raises more risks than I care to have. USB can be wonky at times especially given the variety of devices that are typically connected (and in use) at the same time ... keyboard, mouse, printer, scanner, other drive(s), Flash drives, Facetime camera, iPhone, etc. and are frequently connected or disconnected. I don't trust USB's reliability for RAID. You won't have that issue via Thunderbolt.

I am fairly certain that setting up a RAID array in Disk Utility involves reformatting the drives, which of course will erase all preexisting data on all the drives in the array. One more thing, I also suggest that the format to use for the array be Mac OS Extended Journaled.

All that said, FWIW, I would use an external enclosure that has hardware RAID built in. (Like this one from OWC.) I have never trusted software RAID ... but that's just me.

MartinR wrote:

I would definitely NOT mix connection types with a RAID array,

In addition to everything you said, I would not even use physically separate drives utilizing the same protocol for some of the similar reasons you have already mentioned. I suggest you get a single enclosure which is able to house the number of drives you want. As long as the enclosure supports JBOD, you can use software RAID on each of the drives within the single enclosure.

I am fairly certain that setting up a RAID array in Disk Utility involves reformatting the drives, which of course will erase all preexisting data on all the drives in the array.

Yes, configuring a RAID will destroy all data on those drives.

One more thing, I also suggest that the format to use for the array be Mac OS Extended Journaled.

Agreed. APFS is still a very young file system that tends to become damaged/corrupt where Disk Utility First Aid is unable to repair the volume, which means you will need to erase the entire RAID volume and restore all those files from a backup.

All that said, FWIW, I would use an external enclosure that has hardware RAID built in. (Like this one from OWC.) I have never trusted software RAID ... but that's just me.

In the past I would have recommended a hardware RAID, but unfortunately if anything ever happens to the hardware RAID enclosure, then all of the data on the RAID volume may be inaccessible unless a user can get the enclosure repaired or find another identical enclosure. Repairing a RAID enclosure is probably unlikely and finding an identical working enclosure is likely difficult. But you should also have a good backup of all the data on the RAID so this may not be a real issue, but it will still involve more work & time to restore the data than just moving the drives to another RAID enclosure supporting JBOD and software RAID.

If the RAID enclosure supports JBOD, then Apple software can be used. Most good RAID enclosures will provide a JBOD option in addition to traditional hardware RAID 0/1/5/6 options although the one @MartinR linked does not although some other OWC RAID enclosures do support JBOD. I agree the OWC enclosures are very nice and reliable and I have used the OWC Mercury Elite ones for external storage for years.

I always like to make sure a drive enclosure supports access to the physical drive's SMART health information either through a management interface for the enclosure or by using a third party app such as DriveDx. But I'm probably in the minority for wanting this feature. Some OWC enclosures and adapters may support SMART, but I know some of them do not support SMART.

Another feature I like for my USB3 drives is support for the UASP protocol which allows for faster data transfers which also helps it to work better if USB 1 or USB 2 devices are also connected.

Software RAID on the other hand allows a person to use any other enclosure for those RAID drives since the RAID configuration is located on the drives and not controlled by any hardware enclosure. @MartinR is correct that Apple software RAID can sometimes break for seemingly no reason which would definitely be more likely to break with multiple external drives instead of a single enclosure to house those drives. I know we have used Apple software RAID 1 for many years and sometimes we must rebuild a broken RAID where both drives are completely healthy as best as we can tell by analyzing each drive's SMART health information.

Of course how you are going to utilize this RAID setup may also determine whether you should go with hardware or software RAID since hardware RAID should be less taxing to the host computer and may be faster.

Plus @ad6id needs to make sure to backup that external RAID volume if it holds important and unique data. RAID is not a backup in and of itself. RAID just allows you to keep the storage online when a drive fails (assuming RAID 1, 5, or 6) giving you time to replace a failed drive & rebuild the RAID, hopefully before another drive fails.